1. Field of the Invention
The present invention relates to a flexible printed circuit board that is connected to a circuit substrate and a device to transmit signals, and an electronic device including such a flexible printed circuit board, and particularly to a flexible printed circuit board in which conduction failure does not occur immediately even if the flexible printed circuit board is damaged when the connected circuit substrate and device are attached to and detached from each other, and an electronic device including such a flexible printed circuit board.
2. Description of Related Art
Flexible printed circuit (FPC) boards are used to connect circuit substrates or to connect a circuit substrate and a device such as a disk drive, display panel or memory device. The flexible printed circuit boards include a laminate in which a conductor layer made of a patterned copper foil or the like is laminated on a base material made of an insulating resin such as polyimide, and can be made thinner than connecting cables. In recent years, in order to handle the increased types of signals transmitted between circuit substrates and the miniaturization of devices, multilayer flexible printed circuit boards including a plurality of conductor layers laminated with insulating layers interposed therebetween also have been widely used.
A flexible printed circuit board can include a large number of signal wires, and thus is used to connect circuit substrates fixedly mounted in the housing of an electronic device, or to connect a circuit substrate and an electronic device such as a display panel. In order to make use of the inherent properties of the flexible printed circuit board of is flexibility and capablility of changing shape, the flexible printed circuit board also is used to connect a member that is attachable to and detachable from an electronic device and a circuit substrate that is fixed in the electronic device to be connected to the member.
In the case where a flexible printed circuit board is used to connect a member that is attachable to and detachable from an electronic device and a member that is fixed in the electronic device as described above, the flexible printed circuit board is required to have a length that allows the member that is attachable to and detachable from an electronic device to be sufficiently removed from the electronic device, with an end of the flexible printed circuit board being fixed in the electronic device. On the other hand, in the case of housing a member that is attachable to and detachable from an electronic device in the electronic device, the flexible printed circuit board needs to be housed in the electronic device together with the detachable member. Accordingly, when housed in the electronic device, the flexible printed circuit board folds and is housed together with the member for being housed in the electronic device in a predetermined location of the housing.
The flexible printed circuit board whose flexibility is imparted by forming a wire pattern on a base material made of a resin film has a disadvantage in that it is easily broken when handled. In order to avoid such a disadvantage of the flexible printed circuit board, a technique has been proposed in which copper foil wires through which no signal flows are formed as a dummy pattern in a curved portion of the flexible printed circuit board that is susceptible to breakage and side edges of a portion in which signal wires are formed (JP S60-35412A).
FIG. 11 is a plan view of a conventional flexible printed circuit board disclosed in the patent document mentioned above. A conventional flexible printed circuit board 500 shown in FIG. 11 is composed of a base material 501 made of an insulating resin and a wire pattern 502 made of a copper foil or the like formed on the base material 501. In this conventional flexible printed circuit board 500, dummy wires 505 through which no signal flows are formed in the periphery of a recessed portion 503 formed in the base material 501 and along the inner sides of side portions 504 of a narrow portion of the flexible printed circuit board 500, and aims to improve the strength of the resin base material 501 by using the dummy wires 505 made of a metal foil.
The technique described in the patent document mentioned above is to reinforce a flexible printed circuit board by forming dummy wires made of a metal foil in locations susceptible to breakage, but the copper foil or the like forming the wire pattern is a thin film formed on the resin base material, and it is thus difficult to sufficiently improve the strength of the entire flexible printed circuit board. On the other hand, in order to dispose a thick film member having a sufficient strength, another process is required, and, moreover, the resulting flexible printed circuit board will have increased rigidity, and therefore the property of the flexible printed circuit board that it is capable of flexibly changing shape is compromised.
Also, this technique merely reinforces locations susceptible to breakage by using a metal foil, and thus if a force sufficient to break the reinforced flexible printed circuit board is applied, disconnection or the like will occur due to the flexible printed circuit board being broken, which prevents the electronic device from operating normally, and in the worst case, may cause more severe defects such as damage or burnout of the circuit element due to overcurrent.